Semiconductor device having nitridated oxide layer and method therefor

ABSTRACT

A semiconductor device includes a substrate ( 12 ), a first insulating layer ( 14 ) over a surface of the substrate ( 12 ), a layer of nanocrystals ( 13 ) over a surface of the first insulating layer ( 14 ), a second insulating layer ( 15 ) over the layer of nanocrystals ( 13 ). A nitriding ambient is applied to the second insulating layer ( 15 ) to form a barrier to further oxidation when a third insulating layer ( 22 ) is formed over the substrate ( 12 ). The nitridation of the second insulating layer ( 15 ) prevents oxidation or shrinkage of the nanocrystals and an increase in the thickness of the first insulating layer  14  without adding complexity to the process flow for manufacturing the semiconductor device ( 10 ).

RELATED APPLICATIONS

A related, copending application is entitled “Non-volatile NanocrystalMemory and Method Therefor”, by Rao et al., Ser. No. 11/043,826,assigned to the assignee hereof, and filed concurrently herewith.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor devices, andmore particularly, to a semiconductor device having a nitridated oxidelayer and method therefor.

BACKGROUND OF THE INVENTION

Many semiconductor non-volatile memory arrays require a relatively highvoltage for programming and erasing operations. During manufacture ofthe non-volatile memory arrays, high voltage tolerant transistors thatcan withstand, for example, the relatively high programming and erasevoltages are implemented at the same time as the array. In anon-volatile memory array that relies on nanocrystals for chargestorage, the charge storage layer is formed prior to the formation ofthe high voltage transistor gate oxide. The subsequent formation ofoxide layers may cause further oxidation of the insulating layers.Further oxidation in the non-volatile device may lead to an increase inthe tunnel oxide thickness. Also, further oxidation may cause thenanocrystals to oxidize and shrink. Changing the charge storage layermay lead to the need for higher program and erase voltages. Also,changing the charge storage layer may lead to an undesirable change inprogram and erase threshold voltages.

Therefore, there is a need for a non-volatile memory device having anaccurately controlled charge storage region while also reducing thesteps needed to manufacture the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a portion of asemiconductor device having a tunnel oxide and charge storage layer inaccordance with the present invention.

FIG. 2 illustrates a cross-sectional view of a portion of thesemiconductor device exposed to a nitriding ambient in accordance withthe present invention.

FIG. 3 illustrates a cross-sectional view of a portion of thesemiconductor device after patterning of the charge storage region inaccordance with the present invention.

FIG. 4 illustrates a cross-sectional view of a portion of thesemiconductor device after a gate dielectric is formed adjacent to thepatterned charge storage region in accordance with the presentinvention.

FIG. 5 illustrates a cross-sectional view of a portion of thesemiconductor device after a polysilicon layer is formed in accordancewith the present invention.

FIG. 6 illustrates a cross-sectional view of a portion of thesemiconductor device after gates are formed in the polysilicon layeraccordance with the present invention.

DETAILED DESCRIPTION

Generally, the present invention provides, in one form, a method forforming a semiconductor device comprising: providing a semiconductorsubstrate; forming a first insulating layer over a surface of thesemiconductor substrate; forming a layer of nanocrystals over a surfaceof the first insulating layer; forming a second insulating layer overthe layer of nanocrystals; applying a nitriding ambient to the secondinsulating layer; selectively removing portions of the layer ofnanocrystals and the first and second insulating layers to expose thesurface of the semiconductor substrate; and forming a third insulatinglayer over the exposed surface of the semiconductor substrate.

In another form, the present invention provides a semiconductor devicecomprising: a semiconductor substrate; a first insulating layer formedover a surface of the semiconductor substrate; a patterned layer ofnanocrystals formed over a surface of the first insulating layer; asecond insulating layer formed over the layer of nanocrystals, thesecond insulating layer having a nitrogen content greater than or equalto two (2) atomic percent of the second insulating layer; and a thirdinsulating layer formed on the surface of the semiconductor substrateand not over the first and second insulating layer.

By nitriding the second insulating layer, oxidation of the nanocrystalsand the first insulating layer is reduced, thus reducing, orrestricting, a change in oxide thickness when subsequent oxide layersare formed. Also, using nitridation instead of an oxidation barriersimplifies the manufacturing process.

FIG. 1 illustrates a cross-sectional view of a portion of asemiconductor device 10 having a tunnel oxide 14 and charge storagestack 16 formed on a semiconductor substrate 12. The semiconductorsubstrate 12 may be formed from silicon. A first insulating layer 14 isformed over the substrate 12 which functions as the tunnel oxide for anon-volatile memory cell. The first insulating layer 14 may be silicondioxide, nitrided oxide, or other high-k dielectric. The insulatinglayer 14 may be thermally grown or deposited, and the thickness may beon the order of 50 Angstroms. Charge storage stack 16 includes aplurality of discrete charge storage elements. In the illustratedembodiment, nanocrystals, represented by the small circles in chargestorage layer 16, are used to form the plurality of discrete chargestorage elements. These nanocrystals are typically formed of silicon,but the discrete storage elements may also be formed of clusters ofmaterial consisting of, for example, germanium, silicon carbide, anynumber of metals, or any combination of these. The nanocrystals arepreferably deposited by chemical deposition, but other processes mayalso be used. Other processes for forming nanocrystals includerecrystallization of a thin amorphous layer of silicon and thedeposition of prefabricated nanocrystals. Subsequent to nanocrystalformation, the nanocrystals may be passivated by oxidizing them usingnitrous oxide.

FIG. 2 illustrates a cross-sectional view of a portion of thesemiconductor device 10 exposed to a nitriding ambient. The chargestorage stack 16 includes the nanocrystals 13 surrounded by an oxide 15.Alternately, the charge storage stack 16 may formed by forming aplurality of relatively thin insulating layers, such as an insulatinglayer 17, over one another. After forming charge storage stack 16, thesemiconductor device 10 is exposed to a nitriding ambient. The nitridingambient includes one or more of ammonia, nitrous oxide, atomic nitrogen,or other nitrogen compounds. The process for exposing the semiconductordevice 10 to the nitriding ambient may include one of plasmanitridation, thermal nitridation, or ion nitridation.

The semiconductor device 10 is placed in a processing chamber having oneor more of a plasma source, a thermal source or an ion source.Appropriate chambers are commercially available. In the processingchamber, the semiconductor device is exposed to a plasma 18 to provide anitrogen content of greater than or equal to 2 atomic percent andpreferably between 2 and 10 atomic percent.

FIG. 3 illustrates a cross-sectional view of a portion of thesemiconductor device 10 after patterning of the charge storage region 16and first insulating layer 14 to form a patterned charge storage layer20. A photo resist layer (not shown) is deposited over the chargestorage region 16 and then patterned. Optionally, in another embodiment,the step of nitridation using a nitrogen containing plasma 19 may beaccomplished after patterning instead of before patterning using one ormore of a plasma source, a thermal source or an ion source as describedabove.

FIG. 4 illustrates a cross-sectional view of a portion of thesemiconductor device 10 after a gate dielectric 22 is formed adjacent tothe patterned charge storage layer 20. The gate dielectric 22 may be onethickness throughout or may be different thicknesses to accommodate, forexample, both high voltage transistors and logic circuits.

FIG. 5 illustrates a cross-sectional view of a portion of thesemiconductor device 10 after a polysilicon layer 24 is formed over thepatterned charge storage layer 20 and the gate dielectric 22.

FIG. 6 illustrates a cross-sectional view of a portion of thesemiconductor device 10 after the polysilicon layer 24 is patterned andetched to form gate electrodes. Non-volatile memory cells 23 and 25 arerepresentative of an array of non-volatile memory cells implemented onan integrated circuit. The non-volatile memory cells may be on a“stand-alone memory device or embedded with other circuitry, such as acentral processing unit. Non-volatile memory cells 23 and 25 are formedby selectively etching charge storage layer 20, first insulating layer14, and polysilicon layer 24. The gate electrodes 28 are formed from thepolysilicon layer 24.

Note that the memory array requires additional circuitry, whether on notthe memory array is embedded, to access the memory array, such as rowand column decoders and input/output (I/O) circuits. Some of theseadditional circuits may be exposed to the relatively high programmingand erase voltages and will therefore require thicker gate oxides thanthe circuits not exposed to the higher programming and erase voltages.Transistors 31 and 33 in FIG. 6 represent transistors necessary toimplement the additional circuits. The nitridation of the patternedcharge storage layer 20 prevents oxidation or shrinkage of thenanocrystals in memory cells 23 and 25 and an increase in the thicknessof the first insulating layer 14. The nitidated second insulating layer20 incorporates an oxidation barrier and thus provides a relativelysimple process flow for manufacturing the semiconductor device 10.

Not shown in FIG. 6 but usually included in the formation of transistorsare sidewall spacers and source/drain regions. Typically, the side-wallspacers are formed by deposition of a layer of spacer material, followedby an anisotropic etch of the spacer material. The spacer material istypically nitride. The source/drain regions are typically diffusedadjacent to the gate stack.

While the invention has been described in the context of a preferredembodiment, it will be apparent to those skilled in the art that thepresent invention may be modified in numerous ways and may assume manyembodiments other than that specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention which fall within the true scope of theinvention.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

1. A method for forming a semiconductor device comprising: providing asemiconductor substrate; forming a first insulating layer over a surfaceof the semiconductor substrate; forming a layer of nanocrystals over asurface of the first insulating layer; forming a second insulating layerover the layer of nanocrystals; applying a nitriding ambient to thesecond insulating layer; selectively removing portions of the layer ofnanocrystals and the first and second insulating layers to expose thesurface of the semiconductor substrate; and forming a third insulatinglayer over the exposed surface of the semiconductor substrate.
 2. Themethod of claim 1, further comprising forming a patterned polysiliconlayer over the second insulating layer, wherein the patternedpolysilicon layer forms gate electrodes for a plurality of non-volatilememory cells.
 3. The method of claim 2, wherein the patternedpolysilicon layer is formed over the third insulating layer to form gateelectrodes for a plurality of transistors.
 4. The method of claim 1,wherein applying the nitriding ambient to the second insulating layercomprises applying the nitriding ambient using plasma nitridation. 5.The method of claim 1, wherein applying the nitriding ambient to thesecond insulating layer comprises applying the nitriding ambient usingthermal nitridation.
 6. The method of claim 1, wherein applying nitrogento the second insulating layer comprises applying the nitriding ambientusing ion implantation.
 7. The method of claim 1, further comprising:forming a fourth insulating layer over the second insulating layer; andapplying a second nitriding ambient to the fourth insulating layer. 8.The method of claim 1, wherein forming the second insulating layercomprises forming the second insulating layer by forming a laminate ofinsulating layers.
 9. The method of claim 8, wherein applying thenitriding ambient to the second insulating layer comprises applying thenitriding ambient to each layer of the laminate of insulating layersafter each layer of the laminate of insulating layers is formed.
 10. Themethod of claim 1, further comprising annealing the semiconductor deviceafter applying the nitriding ambient to the second insulating layer. 11.The method of claim 1, wherein the nitriding ambient is applied to thesecond insulating layer after the portions of the layer of nanocrystalsare selectively removed.
 12. The method of claim 1, wherein thenitriding ambient includes one or more of ammonia, nitrous oxide, atomicnitrogen, or other nitrogen compounds.